private PictureBox[] picturebox = new PictureBox[64];
private void button2_Click(object sender, EventArgs e)
{
int i = int.Parse(textBox1.Text);
picturebox[i].BackColor = Color.Green;
}
when the button click i will take a number between 1 to 64 from user and i will change the backcolor of the picturebox which is want by user.
for ex:
int i=3;
pictureBox3.BackColor=color.green;